Gears are commonly used to effect the transfer of power from one rotating element to another. Typically, the teeth of a first gear coupled to a first rotating shaft engage with the teeth of a second gear coupled to a second rotating shaft. The axes of the first and second rotating shafts can be parallel, perpendicular, or at some other angle, depending on the particular needs of the power transfer apparatus. Multiple such gears are often combined to accomplish a desired power transfer between an input shaft and one or more output shafts. This power transfer can effect a desirable transformation of the rotational velocity and torque, as well as a change in the orientation of the axis about which the rotation occurs.
Gears are typically contained within a housing or gear casing. The gear casing provides support for the shafts of the gears, ensuring the engagement of the gear teeth, and prevents debris from fouling the teeth, among other functions. Examples of such gear casings can be found in differentials and power transfer units, among others.
Differentials are typically used in automobiles, trucks, and other wheeled vehicles that utilize paired drive wheels in order to propel the vehicle. The differential operates by transmitting, through a series of engaging gears, a torque from the drive shaft of the vehicle to a pair of axially aligned but independently rotating axles, each of which connects to a driving wheel of the vehicle. Due to the independent rotation of each of the pair of axles, the driving wheels attached thereto can be rotated at non-equal speeds. Such operation is beneficial during, for example, the negotiation of a curve by the vehicle, when the driving wheel located radially outward of the curve must rotate at a faster rotational speed than the driving wheel located radially inward of the curve in order to prevent either dragging of the outer wheel or slipping of the inner wheel.
Power transfer units (PTUs) are used in automobiles, trucks, and other wheeled vehicles in which multiple spaced apart wheeled axes of the vehicle are simultaneously driven by the engine and transmission. As one example, automobiles featuring “all-wheel drive” typically have a front axle driven directly by the drive shaft of the vehicle, and a rear axle driven by a power transfer unit. The power transfer unit operates by transmitting, through a series of engaging gears, a torque from a first shaft located at the front of the vehicle and extending in the width direction of the vehicle, to a second shaft extending perpendicular to the first shaft towards the rear axle of the vehicle, in order to drive the rear axle.
High-viscosity oil is typically contained within the casing of the differential or PTU to adequately lubricate the engaging gears therein. The rotation of the gears churns the oil, thereby ensuring that the meshed gear surfaces are continuously lubricated. Consequently, a pump or other means to move the oil through the casing is typically not required.
Under certain operating conditions, the oil used to lubricate the gears can become heated to temperatures in excess of the recommended operating temperature of the oil. Such operating conditions can include (but are not limited to) high-load operation of the vehicle, high ambient temperatures, restricted airflow over the gear casing, and combinations of the above.
In order to address this problem of overheating the oil, heat exchangers to transfer heat from the oil to an engine coolant loop have been known to be incorporated into the gear casing. An example of such a heat exchanger, shown in Japanese utility model 1991-079052, consists of a flat cooler integrated into the inner chamber of a differential during manufacture. Such a heat exchanger can add considerable cost to the vehicle, and is not needed for all intended uses of the vehicle. It is often not known, at the time that the vehicle is manufactured, whether or not the eventual vehicle owner will operate the vehicle in any of the operating conditions that require cooling of the oil. Furthermore, the addition of such a cooler to the gear casing after the vehicle has already been assembled is very problematic, requiring substantial disassembly of the drivetrain. Thus, there is still room for improvement.